Field of the Invention
The present invention relates to a distance rendering method used for audio signal output and an audio signal output apparatus using the same.
Discussion of the Related Art
Recently, with advances in information technology (IT), a variety of smart devices has been developed. Particularly, smart devices basically provide audio output having various effects. Therefore, various methods for more realistic audio output have been attempted in a virtual reality (VR) environment. Thereamong, a distance rendering method provides more realistic audio in a VR environment by adjusting sense of distance caused by the distance between a sound image and a user through a direct-to-reverberation (D/R) ratio. A conventional general distance rendering method will now be described.
For example, in order to reproduce, through a headphone or an earphone, an effect of localizing a sound image from an arbitrary direction in a VR space, information about the VR space and information about the direction of a sound source are needed. Generally, the information about the direction of the sound source is provided by a user. Accordingly, if a database (DB) of head-related transfer functions (HRTFs) measured in all directions is present, a sound image may be localized in a desired direction with reference to the provided direction information. Further, if a head tracker is used together with the DB, the sound image may be localized in a fixed direction regardless of movement of the head of a user. Generally, information about a space may be acquired by measuring a room impulse response (simply, “room response”) measured in the corresponding space. However, since the room response cannot be actually measured in a “VR space”, the room response may be artificially generated using a room impulse response modeling (hereinafter, simplified to “room modeling”) method. If the generated room response is filtered together with an input signal, a user may perceive a spatial sense or reverberation for the measured space.
A user (listener) in an arbitrary space may move and the distance between a sound image and the user differs according to a moved location. This means that reverberant sound as well as the magnitude of a sound source to which the listener listens becomes different. Accordingly, a room response characteristic varies according to movement of a user.
FIG. 1 illustrates a conventional normal room response characteristic. Generally, the room response characteristic is divided into a direct part 11, an early reflection part 12, and a late reverberation part 13 as illustrated in FIG. 1. The direct part 11 is associated with the magnitude of a sound source and intelligibility. The early reflection part 12 and the late reverberation part 13 are associated with a spatial sense and reverberation of a sound source.
A conventional distance rendering method adjusts sense of distance through signal processing by modifying a room response. Generally, the conventional distance rendering method uses a D/R ratio. The D/R ratio is expressed by Equation (1).
                              D          ⁢                      /                    ⁢          R                =                                            P              D                                      P              R                                =                                                                      ∫                  0                                      t                    1                                                  ⁢                                                                            h                      2                                        ⁡                                          (                      t                      )                                                        ⁢                  d                  ⁢                                                                          ⁢                  τ                                            ⁢                                                                                                                    ∫                                      t                    1                                    t                                ⁢                                                                            h                      2                                        ⁡                                          (                      t                      )                                                        ⁢                  dt                                            ⁢                                                                                                      (        1        )            
In Equation (1), h(t) denotes a room response and t1 denotes time until a response of the direct part 11 is received. As indicated by Equation (1), the D/R ratio is controlled by the ratio of energy of the direct part 11 to energy of the early reflection part 12 and the late reverberation part 13.
For reference, FIG. 2A illustrates a room response characteristic when the distance between a user and a sound image is not changed. The room response characteristic may be indicated by distinguishing between a direct part 11a and an early reflection and late reverberation part 14a, based on t1 of Equation (1).
For example, if it is desired that a relative distance between the user and the sound image be short, a gain of a direct part 11b of the room response may be adjusted to be increased and a gain of an early reflection and late reverberation part 14b may be adjusted to be decreased as illustrated in FIG. 2B. Then, a sound source sounds bigger and a reverberant sound sounds smaller.
On the contrary, if it is desired that a relative distance between the user and the sound image be increased, a gain of a direct part 11c may be adjusted to be decreased and a gain of an early reflection and late reverberation part 14c may be adjusted to be increased, as illustrated in FIG. 2C. Then, the sound source sounds smaller and the reverberant sound sounds bigger.
However, if it is desired that the distance between the sound image and the user be very close, a gain of the direct part is remarkably increased relative to a gain of the early reflection and a gain of the late reverberation part. In addition, if a remarkably modified gain is applied to a room response, the user may not feel reverberation even though the response has been measured in a space having reverberation. That is, a spatial sense is decreased as if the response is measured in an anechoic room (or dead room) and thus a spatial characteristic varies.
Moreover, a correlation between left and right ears (simply, binaural correlation) according to change in distance cannot be adjusted only by adjustment of the D/R ratio. Accordingly, if the distance between the sound image and the user is longer, a gain of the sound image is decreased and the binaural correlation is also decreased.
Accordingly, a new method capable of maintaining a spatial characteristic even when a relative distance between the user and the sound image is changed is needed. In addition, a new method capable of maintaining a binaural correlation characteristic based on the spatial characteristic even when a relative distance between the user and the sound image is changed is needed. That is, a binaural correlation value should be changed according to change in distance in order to maintain a spatial characteristic.